Tuchman, M, L, US EPA Great Lakes National Program Office, Chicago, USA, tuchman.marc@epa.gov

Warren, G, J, US EPA Great Lakes National Program Office, Chicago, USA, warren.glenn@epa.gov

Rockwell, D, C, US EPA Great Lakes National Program Office, Chicago, USA, rockwell.david@epa.gov

RESPONSE OF SILICA CONCENTRATION AND DIATOM ABUNDANCES TO REDUCTIONS IN PHOSPHORUS LOADING TO LAKE MICHIGAN

During most of the last century the Laurentian Great Lakes were subject to increased anthropogenic loadings of phosphorus. The production response of diatoms to these increases resulted in a dramatic decline in the silica reservoir in Lake Michigan, and concomitant silica limitation of summer diatom growth. Decreases in phosphorus loading to Lake Michigan achieved in the past thirty years now appear to be reversing this trend. Long term (1983-2000) records of silica concentration from the vernal unstratified period show that these concentrations have increased steadily over the past eighteen years. Silica during the summer stratified period (August-September) remained relatively constant at low concentrations through 1991, and increased notably after that, suggesting a release from silica limitation of diatom communities during the mid and late 1990's. Diatom cell counts show the expected response to increased silica concentrations, exhibiting variable but pronounced increases after about 1992. We believe this provides the first clear evidence that Lake Michigan is responding to the vigorous phosphorus abatement programs of the past thirty years.